1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device driven by a thin film transistor.
2. Description of the Related Art
Driving frequency has recently been made higher to improve motion picture quality in an active matrix type liquid crystal display device driven by a thin film transistor. Accordingly, it is essential to lower the resistance of signal lines. Further, the active matrix type liquid crystal display device is used for television receivers of a large size screen. Accordingly, reduction in cost is demanded and it is strongly required to keep production cost low.
To decrease the resistance of the signal line, a metal material of low electrical resistivity such as aluminum or copper may be used as a main wiring material. However, aluminum or copper cannot be used alone as the wiring material. For example, since aluminum cannot be in direct contact electrically with a transparent conductive film (generally, oxide mainly comprising indium) in a different layer and, accordingly, it is necessary to laminate a connection layer of molybdenum or titanium at the boundary between aluminum and the transparent conductive film. Further, since aluminum diffuses unnecessarily in silicon for forming a thin film transistor, it is necessary to laminate a barrier layer of molybdenum or titanium also to the boundary between aluminum and silicon. On the other hand, since copper also diffuses unnecessarily into silicon, a barrier layer has to be laminated at the boundary between copper and silicon. Further, since copper has weak adhesion force with an underlying layer, an adhesive layer has to be laminated at the boundary between copper and the underlayer.
As described above, when aluminum or copper is used as a material for signal lines of thin film transistor driven liquid crystal display devices, a barrier layer or an adhesive layer of molybdenum or titanium has to be laminated inevitably.
However, molybdenum and titanium not only contribute scarcely to the lowering of the resistance in the signal line but also are expensive compared with aluminum or copper. That is, they involve a problem not contributing to the reduction of the cost. Further, since a step of laminating the barrier layer or adhesive layer is necessary, this takes much time for film deposition treatment and requires expensive sputtering apparatus disposed additionally. Accordingly, this results in a drawback of increasing the installation investment in mass production factories.
An example of an aluminum alloy that can be used as a single layer signal line is disclosed in “Resource and Material” Vol. 120 (2004), p. 310. When the aluminum alloy is used for connection wiring between a thin film transistor and a pixel electrode in a liquid crystal panel, the productivity is improved. However, the lower limit of the electrical resistivity of the aluminum alloy is 3.7 μΩcm and it is difficult to further lower the electrical resistivity. Further, when the aluminum alloy is used alone, it cannot cope with a problem regarding long time reliability of signal lines such as stress migration.
WO 2006/025347A1 describes an example of a copper alloy that can be used as a single layer for a signal line of a liquid crystal display device. The absolute value of a Gibbs Free Energy of Formation of oxide or enthalpy of formation of oxide of additive metal element of the copper alloy is larger than that of copper, and the diffusion coefficient of the additive metal element in copper is larger than the self-diffusion coefficient of copper. The alloy film is annealed at 400° C., an oxide of an additive metal element is formed at a boundary with a silicon oxide as an underlayer. This improves the adhesion with the underlayer and the electrical resistivity which was higher before annealing can be lowered.
In WO 2006/025347A1, annealing is conducted at 400° C. However, the temperature in production process of a thin film transistor-driven liquid crystal display device using amorphous silicon is at about 300° C. or lower. Accordingly, it is necessary that the electrical resistivity is lowered sufficiently even by annealing at 300° C. An aimed value for the specific electrical resistivity is below 3.7 μΩcm which is the electrical resistivity of the aluminum alloy that can be used as a single layer described in “Resource and Material” cited above. The present inventors have made an experiment of conducting annealing at 300° C. As a result, the electrical resistivity of the copper alloy formed on the silicon oxide was not lowered sufficiently compared with that before annealing. If it is intended only to lower the electrical resistivity, the content of the additive metal element may be decreased, but this lowers the adhesion with the underlayer.
The present invention intends to provide, in a thin film transistor-driven liquid crystal display device, an interconnect structure capable of sufficiently lowering the electrical resistivity even by annealing at 300° C. and having high adhesion with an underlayer.